have had a/c charge checked and a/c comp.turns off & on( 50-55deg @vent)in truck never gets cold pls help

Sounds like the system is either undercharged or overcharged. Put a good set of R134a gauges on it. Your outlet vents should be down to about 40 degrees with an outside temp of 80. (Hint: if the system is a bit undercharged, the evaporator could sometimes ice up, which would block the flow of air.)